April 17. 2013 11:45AMWASHINGTON (Reuters) - U.S. Capitol Police cleared parts of two U.S. Senate office buildings over suspicious letters and a package discovered there Wednesday morning, but just before 1 p.m. announced the Hart Senate Office Building has reopened after the substance found there tested negative.

Parts of the Russell and Hart buildings were checked as officials investigated, a U.S. Capitol Police spokesman said.

Officials were looking into a suspicious package in the Hart building. They also looked into suspicious envelopes in the Russell building and questioned an individual in that matter, the spokesman said.

A spokeswoman for U.S. Sen. Kelly Ayotte said the New Hampshire Republican's office has not been affected by the security scare.

The Russell Building is where Ayotte's office is located. The area that was cleared was not near Ayotte's office and her staff says all is OK.

The office of U.S. Sen. Jeanne Shaheen (D-NH) office is in the Hart Building. Spokesman Shripal Shah said Capitol Police asked staff members to remain in the office while investigating the suspicious packages. The advisory was only temporary and police told the staff they could resume normal activities before 1 p.m.

On Tuesday U.S. authorities intercepted a letter sent to U.S. Senator Roger Wicker of Mississippi that preliminary tests showed contained the deadly poison ricin.

Also on Tuesday, a letter addressed to President Barack Obama containing a suspicious substance was received at a White House mail screening facility, the U.S. Secret Service said on Wednesday.

The facility is remote and not located near the White House itself, spokesman Edwin Donovan said in a statement.

"The Secret Service is working closely with the U.S. Capitol Police and the FBI in this investigation."

In Oklahoma City Wednesday, police used a robot to investigate a suspicious van parked outside City Hall, and while no threat was found, officials were on high alert in the wake of the Boston Marathon bombings.

City Hall and surrounding buildings were evacuated while police inspected a U-Haul van parked outside the government buildings. Police said the van had been stolen and that it was reported as unattended by a nervous citizen.

The incident happened just two days before the anniversary of the April 19, 1995, truck bombing of a federal government building in Oklahoma City that killed 168 people and injured more than 650 others.

Officials said they are always vigilant for any potential threat, but nerves are on edge following the bombs in Boston at the marathon finish line on Monday that killed three people and injured 176 others.

"People are very sensitive right now because of the Boston bombing," said Oklahoma City Police spokeswoman, Sgt. Jennifer Wardlow.